Propagation of short fatigue cracks

Fatigue crack propagation in engineering materials has been the subject of considerable research, and extensive review articles have appeared over the past several years. Most of these investigations focused on the behaviour of ‘long’ fatigue cracks, even though the characteristics associated with the extension of small cracks in metals and alloys remain relatively unexplored, despite their unquestionable importance from an engineering standpoint. In this review, the mechanics and micromechanisms of the subcritical growth of short fatigue cracks are examined, and aspects of their propagation behaviour are contrasted with those of long cracks in terms of fracture mechanics, microstructure, and environment. Cracks are defined as being short (i) when their length is small compared to relevant microstructural dimensions (a continuum mechanics limitation), (ii) when their length is small compared to the scale of local plasticity (a linear elastic fracture mechanics limitation), or (iii) when they are simply physically small (e.g. ≤ O. 5-1 mm). Since all three types of short flaw are known to propagate faster. than (or at least at the same rate as) corresponding long fatigue cracks subjected to the same nominal driving force, current defect tolerant fatigue design procedures which utilize long crack data can, in certain applications, result in overestimates of lifetimes. The characteristics of the short crack problem are critically reviewed in the light of the influences of local plasticity, microstructure, crack tip environment, growth mechanisms, crack driving force, and the premature closure of the crack.